In general, epitaxial growth can be described as the growth or deposition of one or more thin crystalline layers (epitaxial layers) of various semiconductor materials on a crystalline substrate to form an epitaxial structure. Two types of epitaxial growths are homoepitaxial growth and heteroepitaxial growth. Homoepitaxial growth is when the substrate and epitaxial layers deposited on the substrate are made of the same material. Heteroepitaxial growth is when the substrate and the epitaxial layers deposited on the substrate are made of different materials. For heteroepitaxial growth, the atomic structures of the substrate and a first epitaxial layer must be similar in order to avoid major defects in the crystalline structure of the epitaxial layers due to mismatches between the lattice structures of the substrate and the first epitaxial layer. To overcome this limitation, a thin nucleation layer may be deposited on the substrate before depositing the first epitaxial layer.
The epitaxial structure may include any number of epitaxial layers. A single epitaxial layer of a desired material may be deposited on the substrate, where the epitaxial layer can be used as a seed to grow more of the desired material. For example, a single layer of gallium nitride (GaN) may be deposited on the substrate and later used as a seed to grow additional GaN epitaxial layers. On the other hand, multiple epitaxial layers may be deposited on the substrate in order to produce an epitaxial structure suitable for use in fabrication of an opto-electronic or electronic device, such as a high electron mobility transistor (HEMT) or a metal-insulator-semiconductor field effect transistor (MISFET).
For GaN epitaxial structures, one common material used as the substrate is sapphire. Sapphire is relatively inexpensive; however, it has properties such as poor thermal conduction, that are undesirable for many devices. Therefore, it may be desirable to separate the epitaxial layers from the sapphire or other substrates. Thus, there remains a need for a GaN epitaxial structure and a method of growing a GaN epitaxial structure, wherein the epitaxial layers can be separated from the substrate after growth.